| J58 | |
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| J58 Jet engine on display at Duxford, UK | |
| Type | Turbojet |
| Manufacturer | Pratt & Whitney |
| First run | 1958 |
| Major applications | Lockheed A-12 Lockheed SR-71 |
The Pratt & Whitney J58 (company designation JT11D) was a variable cycle turbojet aircraft engine used on the Lockheed A-12, and subsequently on the YF-12 and SR-71 Blackbird aircraft. It was essentially a hybrid turbojet/ramjet engine.
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Design and development
The J58 was initially developed for the US Navy to power the planned version of the Martin P6M jet flying boat.[1] Upon cancellation of this aircraft, it was selected by Convair and Lockheed for their supersonic projects. Other sources link its origin to the USAF's requirement for a powerplant for the WS-110A, the future XB-70 Valkyrie.[2] The J-58 produced 32,000 lbf (142 kN) of thrust. It was the first engine to be able to operate on afterburner for extended periods of time, and the first engine to be flight-qualified by the United States Air Force for Mach 3. A major feature of the J58 was the conical spikes in the variable-geometry inlets, which were automatically moved fore and aft by an Air Inlet Computer. The spike altered the flow of supersonic air, ensuring subsonic airflow at the engine inlet. The conical spikes are locked in forward position below 30,000 feet. Above that altitude they are unlocked. Above Mach 1.6 airspeed they are retracted approximately 4 cm (1-5/8 inch) per 0.1 Mach, up to total of about 66 cm (26 inches).
The J58 was a variable cycle engine which functioned as both a turbojet and a fan-assisted ramjet. Bypass jet engines were rare at the time, but Ben Rich later described the engine as "bypass jet engine by air withdrawal".[3] At Mach 3.2, 80% of the engine's thrust came from the ramjet section, with the turbojet section providing 20%.[4] At lower speeds, the J58 operated as a pure turbojet.
The engine was started with an AG330 starter cart, with two Buick Wildcat V8 internal combustion engines with a common driveshaft. The cart spun the J58 to 3,200 rpm before the turbojet cycle could start. Later, a conventional start cart was used.[citation needed]
The engine's high operating speeds and temperatures required a new jet fuel, JP-7. Its reluctance to be ignited required triethylborane (TEB) to be injected into the engine to ignite it, and to ignite the afterburner in flight; above -5 °C TEB spontaneously ignites in contact with air. Each engine carried a nitrogen-pressurized sealed tank with 600 cm³ of TEB, sufficient for at least 16 starts, restarts, or afterburner lights; this number was one of the limiting factors of SR-71 endurance, as after each air refueling the afterburners had to be reignited.[5] When the pilot moved the throttle from cut-off to idle position, fuel flowed into the engine, and shortly afterwards a 50 cm³ shot of TEB was injected into the combustion chamber, where it spontaneously ignited and lit the fuel with a green flash. In some conditions, however, the TEB flow was obstructed by coking deposits on the injector nozzle, hindering restart attempts. Refilling the TEB tank was a perilous task; the maintenance crew wore silver fire suits.[6] Conversely, the JP-7 fueling was so safe that some aircraft maintenance was permitted during filling. The chemical ignition was chosen instead of a conventional igniter for reliability reasons, and to reduce mechanical complexity. The TEB tank is cooled with fuel flowing around it, and contains a disk that ruptures in case of overpressure, allowing TEB and nitrogen to discharge into the afterburner.
The fuel flowing into the engine is used as a coolant to cool the engine, hydraulic fluid, oil, TEB tank, afterburner nozzle actuator control lines, air conditioning systems, and the parts of the airframe subjected to aerodynamic heating.
The engine lubricant was a silicone-based grease. It was solid at room temperature, and was preheated prior to engine start.
Turbo-ramjet design
The J58 is a hybrid jet engine: effectively, a turbojet engine inside a fan-assisted ramjet engine. This was required because turbojets are inefficient at high speeds but ramjets cannot operate at low speeds. To resolve this, the airflow path through the engine varied, depending on whether ramjet or turbojet operation was more efficient, thus the term "variable cycle". To create this effect, at speeds over 2000 mph the nose cone of the engine was pushed about 2 inches forward to improve the air flow in the ramjet cycle.
Air is initially compressed and heated by the shock wave cones, and then enters 4 stages of compressors, and then the airflow is split:[7] some of the air enters the compressor fans ("core-flow" air), while the remaining flow bypasses the core to enter the afterburner. The air continuing through the compressor is further compressed before entering the combustor, where it is mixed with fuel and ignited. The flow temperature reaches its maximum in the combustor, just below the temperature where the turbine blades would soften. The air then cools as it passes through the turbine and rejoins the bypass air before entering the afterburner.
At around Mach 3, the initial shock-cone compression greatly heats the air, which means that the turbojet portion of the engine must reduce the fuel/air ratio in the combustion chamber so as not to melt the turbine blades immediately downstream. The turbojet components of the engine thus provide far less thrust, and the Blackbird flies with 80% of its thrust generated by the air that bypassed the majority of the turbomachinery undergoing combustion in the afterburner portion and generating thrust as it expands out through the nozzle and from the compression of the air acting on the rear surfaces of the spikes.
Applications
Specification of J58-P4
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This aircraft engine article is missing some (or all) of its specifications. If you have a source, you can help Wikipedia by adding them. |
Data from [8]
General characteristics
- Type: afterburning variable cycle turbojet/ramjet
- Length:
- Diameter:
- Dry weight:
Components
- Compressor:
- Fuel type: JP-7
Performance
- Maximum Thrust: 34,000 pounds-force (150 kN) (wet), 25,000 pounds-force (110 kN) (dry)
- Specific fuel consumption: 1.9 lb/(lbf-h) (wet), 0.9 lb/(lbf-h) (dry)
- Thrust-to-weight ratio:
- Core air flow: 450 lb/s, (200 kg/s)
References
- ^ [1]
- ^ Goodall, James and Jay Miller. "Lockheed's SR-71 'Blackbird' Family A-12, F-12, M-21, D-21, SR-71". Hinckley, England: AeroFax-Midland Publishing, 2002. ISBN 1-85780-138-5.
- ^ The Heart of the SR-71 "Blackbird": The mighty J58 engine
- ^ Pratt & Whitney J58 Turbojet, Hill Aerospace Museum
- ^ http://www.netwrx1.com/skunk-works/v05.n717
- ^ http://yarchive.net/air/sr71.html
- ^ The heart of the SR-71 : the J-58 engine. Evolutions
- ^ Military Turbojet/Turbofan Specifications
External links
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Wikimedia Commons has media related to: Pratt & Whitney J58 |
- P&W J58 page
- EngineHistory.org P&W J58 images
- SR-71 Online - J58 Engine Photos
- U. S. Patent 3,344,606, "Recover Bleed Air Turbojet," Robert B. Abernethy
- U. S. Patent 3,477,455, "Supersonic Inlet for Jet Engines," David H. Campbell.
- J58 page on mzak.cz (cs)
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